Field of the Invention
The present invention relates mainly to a ghost cancelling system, and particularly to such a system in which a divergence of a ghost cancelling circuit is avoided during the transient time for the channel selection so that the ghost signal can be reliably eliminated.
A video signal transmission path including a ghost originating source is expressed by a simple block diagram as shown in FIG. 1. In FIG. 1, an original video signal S(w) is supplied to an adder 11, and also the video signal S(w) is supplied through a ghost originating source 12 with a transmission function expressed by G(w) to the adder 11. Accordingly, from the adder 11 is delivered a video signal such as EQU S(w) (1+G(w))
which includes a ghost signal component shown by S(w)G(w).
In order to eliminate such a ghost signal component, an imitation circuit for imitating the transmission function of the ghost originating source 12 is provided and an input video signal including ghost is used to form a ghost imitating signal used as a ghost cancelling signal. Then, the ghost cancelling signal is subtractively added to the input video signal so that the ghost is eliminated. For such a ghost cancelling circuit, there are known two types of circuits such as shown in FIGS. 2A and 2B.
FIG. 2A shows a ghost cancelling circuit known as a feedback type one. In FIG. 2A, a video signal including ghost is passed through a subtraction type adder 13, while the output signal of the adder 13 is supplied to a ghost imitation circuit 14 to form a ghost cancelling signal which is then fed back to the adder 13 for being subtracted from the input video signal.
FIG. 2B shows a ghost cancelling circuit known as a feed-forward type one. In FIG. 2B, an input video signal including ghost is supplied to the subtraction type adder 13, while the input video signal is supplied to the ghost imitation circuit 14 to form a ghost cancelling signal which is fed forwardly to the adder 13 for being subtracted from the input video signal.
In the ghost cancelling circuit shown in FIG. 2B, it is known that a secondary ghost is generally produced in the ghost cancelling process. As a result, in the prior art ghost cancelling system, the feedback type ghost cancelling circuit as shown in FIG. 2A has been more often adopted. FIG. 3 shows an embodiment of such a known feedback type ghost cancelling system.
In the ghost cancelling system shown in FIG. 3, a video signal including ghost, which is detected by a synchronous detector (not shown), is supplied through an input terminal 1 to an adder 2. To the adder 2 is also supplied a ghost imitation signal as a ghost cancelling signal from a transversal filter which will be described later. Then a ghost cancelled-out video signal appears at an output terminal 3. The video signal obtained at the output side of the adder 2 is fed to a delay circuit 4. The delay circuit 4 is composed of a plurality of stages (e.g., 15 stages) of delay units each having a delay time corresponding to a signal sampling period (e.g., 100 nanoseconds[ns]) with n taps being led out from respective stages. The outputs from the n taps of the delay circuit 4 are supplied to a multiplier type weighting circuit 5 so that weighting functions are multiplied thereto respectively. All of the outputs therefrom are supplied to an adder 6 to generate a ghost cancelling signal therein.
Weighting functions for the weighting circuit 5 are generated in an analog accumulative adder 7. The detection of a ghost component is achieved by supplying the output signal of the adder 2 to a ghost detector circuit 8. As a ghost level detecting period, a period which is included in the standard television signal and not affected by the video signal as long as possible is selected. A vertical synchronizing signal period is generally selected as such a ghost detecting period. In general, as shown in FIG. 4, the period of H/2 from the front edge VE of a vertical synchronizing signal to an equalizing pulse HE is selected as the detecting period as well known in the prior art. The signal level during the detecting period is differentiated and weighting functions are formed therefrom, and the tapped outputs of the delay line are weighted in proportion to the differentiated level. For example, at the high frequency stage, when a ghost having a delay time .tau. and a phase difference .zeta.(.zeta.=w.sub.c .tau. where w.sub.c is an angular frequency of the video carrier signal at the high frequency stage) is included supposing that the phase difference .zeta. is around 45.degree. , a video signal having a waveform as shown in FIG. 5A is obtained during the ghost detecting period. This signal is differentiated and inverted in its polarity, so that a differentiated waveform as shown in FIG. 5B is obtained. Since this differentiated waveform can be approximately regarded as the impulse response of the ghost signal, the weighting functions are formed in proportion to the level of this differentiated signal. Accordingly, the differentiated waveform of a video signal during the ghost detecting period is obtained from the ghost detecting circuit 8 and this differentiated signal is supplied to a demultiplexer 9 successively. The demultiplexer 9 is similar to the delay circuit 4 and composed of a plurality of stages of delay units each having a delay time corresponding to the signal sampling period with n taps being led out from respective stages. The outputs of the n taps therefrom are fed to the analog accumulative adder 7 respectively.
The delay circuit 4, weighting circuit 5 and adder 6 are combined to form a transversal filter, and this transversal filter is inserted into the feedback loop to form a so-called inverse filter so that the ghost component in the input video signal can be eliminated. In this case, a distortion of the waveform during a period of 1/2H after the front edge of a vertical synchronizing signal is detected to determine the weighting functions. Thereafter, if the ghost component still remains uncancelled in the output video signal, the remaining ghost component is detected again in the detector 8 and the analog accumulative adder 7 operates to decrease the remaining ghost component.
However, in such a feedback type ghost cancelling system, there is the possibility that the ghost cancelling operation of the ghost cancelling circuit may diverge in the ghost cancelling process during the transient time of the channel selection. In more detail when a received channel is changed over from one to another, it happens that values corresponding to the weighting functions relating to the preceding channel remain in the analog accumulative adder 7 and the remaining weighting functions are meaningless at all with respect to a newly received channel having the different angular frequency w.sub.c of the video carrier. Therefore, during the transient time when a new channel is selected, the adder 2 delivers a video signal including a false ghost component different from the original or real ghost component. Such a false ghost is different in tendency from the real ghost, and therefore it is apprehended that the ghost cancelling circuit may diverge in the ghost cancelling process.
To prevent such a divergence, it can be proposed that during the channel selection, the weighting functions held or memorized in the analog accumulative adder 7 are cleared or reset to a reference DC value. However, in order to provide such a reset circuit, it is required to provide resetting means at the respective n taps of the weighting circuit 5 with the result that the whole circuit becomes quite complicated.
In the description above the divergence of the circuit is taken as a problem during the channel selection, but such a problem also arises when a continuous noise of a so-called ignition noise and so on is detected, because such a noise is detected as a false ghost which is different from the actual ghost, so that there is the possibility that the incorrect and meaningless weighting may cause the divergence of the ghost cancelling circuit.